1. Field of the Invention
This invention relates to a method and apparatus for deployment of a stent or scaffold in the treatment of coronary and peripheral artery disease.
2. Description of the State of the Art
This invention relates generally to methods of treatment with radially expandable endoprostheses that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A scaffold is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent vasospasm and acute closure, as well as tack up dissections. Stents also reduce restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of a scaffold or scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffold gets its name because it physically holds open and, if desired, expands the wall of a passageway in a patient. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site.
Stents are typically implanted by use of a catheter which is inserted at an easily accessible location and then advanced through the vasculature to the deployment site. The stent is initially maintained in a radially compressed or collapsed state to enable it to be maneuvered through a body lumen. Once in position, the stent is usually deployed either automatically by the removal of a restraint, actively by the inflation of a balloon about which the stent is carried on the deployment catheter, or both.
In reference to balloon expandable stents, the stent is mounted on and crimped to the balloon portion the catheter. The catheter is introduced transluminally with the stent mounted on the balloon and the stent and balloon are positioned at the location of a lesion.
The balloon is then inflated to expand the stent to a larger diameter to implant it in the artery at the lesion. An optimal clinical outcome requires correct sizing and deployment of the stent.
An important aspect of stent deployment is the rapidity with which the stent is expanded. For balloon deployed stents, this is controlled by the balloon inflation rate. Inflation is usually achieved through manual inflation/deflation devices (indeflators) or an indeflator unit that possesses some automation.
Stents made from biostable or non-degradable materials, such as metals that do not corrode or have minimal corrosion during a patient's lifetime, have become the standard of care for percutaneous coronary intervention (PCI) as well as in peripheral applications, such as the superficial femoral artery (SFA). Such stents, especially antiproliferative drug coated stents, have been shown to be capable of preventing early and later recoil and restenosis.
It has been recognized for metal stents that slower inflation is better. During inflation, kinetic effects create nonequilibrium conditions. For example, the friction of the balloon against the stent is affected by inflation rate. A fast inflation/deflation cycle can result in higher levels of stent recoil. Inflation speed affects the uniformity of stent deployment along its length. Often, the distal and proximal ends of the balloon inflate first. A dumbbell or dog bone shape is created which exerts a net inward force on the stent. Consequently, fast stent inflation may lead to more stent shortening. Ideally, the stent is deployed uniformly, with an even spreading of the struts around the periphery. Fast deployment is believed to increase the likelihood of struts being clustered together in sections and over expanded in others.
Additionally, there are many potential reasons why inflation speed affects stent expansion. Catheter balloons can be folded in a non-uniform way. Balloon folds can get stuck or “caught” on stent struts. The lesion environment is rarely uniform, or with a perfectly concentric plaque. Lesions are typically eccentric, sometimes with fibrous or calcified focal regions. Consequently, the resistance to radial expansion of the balloon may be greater in certain directions resulting in an eccentric deployment. Lesion tissues can also have viscoelastic properties such that high inflation rates may result in more tissue damages versus slow stretching of the tissue. Lastly, in clinical practice, the process of stent implantation can become routine. Such familiarity often leads to shorter procedural times which tend to beneficial for the patient except for steps, such as stent inflation, where faster is not always better.
In order to effect healing of a diseased blood vessel, the presence of the stent is necessary only for a limited period of time, as the artery undergoes physiological remodeling over time after deployment. A bioresorbable stent or scaffold obviates the permanent metal implant in vessel, allowing late expansive luminal and vessel remodeling, leaving only healed native vessel tissue after the full resorption of the scaffold. Stents fabricated from bioresorbable, biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers can be designed to completely absorb only after or some time after the clinical need for them has ended. Some or all of the reasons for a slower inflation rate for metallic stent may also apply to balloon expandable polymer stents or scaffolds. Most importantly, due to the viscoelastic properties of high molecular weight polymers, these materials tend to become more rigid when deformed at a faster rate, which makes it critical to control the inflation rate of a polymeric stent to prevent any potential formation of cracks and broken struts.